Using a high current density electric arc to form a plasma for depositing a coating upon a substrate, within an evacuated chamber, through the evaporation of source material, is known in the art as the "physical vapor deposition arc process." The source material may be supplied from a solid cathode arranged in the evacuated chamber spaced apart from the substrate. The electric arc is formed between the cathode and an anode connected in circuit with a power supply located external of the chamber. The high current density arc forms a plasma in the cathode region of the arc discharge which includes atoms, molecules, ionized atoms and ionized molecules of the "cathode evaporation surface." The "cathode evaporation surface" is that surface of the cathode to which the electric arc attaches. Coating compounds may be deposited and/or formed on the substrate by introducing reactive gases into the chamber adapted to react with the metal vapor in the plasma.
The physical vapor deposition arc process as it is conventionally known and practiced is shown and described in U.S. Pat. Nos. 3,625,848; 3,783,231; 3,836,451; and 3,793,179, respectively. In accordance with conventional practice and as taught in the above-mentioned patents, a reactive gas can be introduced into the evacuated chamber to react with the source material for forming a coating compound but is otherwise considered irrelevant to the process. Moreover, the method of gas introduction and location in the arc chamber is not considered by those skilled in the art to have significance.
It has been discovered in accordance with the present invention that the deposition of source material in the physical vapor deposition arc process may be controlled by introducing a reactive or inert gas into the evacuated chamber in a predetermined manner as will be elaborated upon hereafter. It has been further discovered that the reactive or inert gas may be introduced into the evacuated chamber in a manner which provides adjustable control over the properties and characteristics of the coating. In fact, the method of the present invention can be used to control the crystal orientation of the deposited polycrystalline coating compound. Control over the crystal orientation and residual stress of titanium nitride (TiN) coating using a solid titanium cathode, or zirconium nitride (ZrN) coating using a solid zirconium cathode, and nitrogen-reactive gas as the source materials forms the basis of corresponding patent applications, U.S. Ser. No. 781,459 filed Sept. 30, 1985, and its continuation-in-part application, Ser. No. 905,510, filed concurrently herewith and entitled "Titanium Nitride and Zirconium Nitride Coating Compositions, Coated Articles and Method of Manufacture" herein incorporated by reference. In addition to the control provided over the characteristics of the coating, the process and apparatus of the present invention improve the operation of the physical vapor deposition arc process by maximizing confinement of the arc to the "cathode evaporation surface" and minimizing the potential of the arc to extinguish during operation. Furthermore, the process and apparatus of the present invention permits continuous, stable operation of the apparatus for depositing a coating from the cathode over an extended time period of up to three or four times greater than that operated with the prior art.